Dark trace cathode-ray tube with improved erasing means



Nov. 26, 1968 a. J. HART `l-:T AL

DARK TRACE CATHODE-RAY TUBE WITH IMPROVED" ERASING MEANS Nov. ze, 196sB,J,HART ETAL 3,413,505

DARK TRACE CATHODE-RAY TUBE WITH IMPROVED ERASING MEANS J-nz/enzrsx dlJHQT, ZQr REU/'aldi United States Patent O 3,413,505 DARK TRACECATHODE-RAY TUBE WITH IMPROVED ERASING MEANS Bill J. Hart, Bloomington,and Earl R. Ewald, Normal, Ill., assignors to National Union ElectricCorporation, Bloomington, Ill., a corporation of Delaware Filed May 31,1966, Ser. No. 554,025 10 Claims. (Cl. 313-91) ABSTRACT F THE DISCLOSUREThis disclosure deals with a dark trace cathode-ray tube including ascotophor screen and improved means for erasing an image from thescreen. The era-sing means is located behind the screen and includes anannular heating coil and an annular generally parabolic reflector. Thecoil is located approximately at the focal line of the reflector, andheat generated :by the coil is reflected by the reflector to the screento erase an image from the screen.

Disclosure A dark trace cathode-ray tube includes a bulb which forms asealed enclosure, an electron gun within the enclosure, and a screen,also within the enclosure, which has a trace or image formed thereonwhen electrons from the gun impinge upon it. The screen retains theimage until the image is erased in some manner, erasure usually beingaccomplished by heating the screen. One apparatus for heating the screento obtain erasure has included a tungsten filament mounted within theenclosure adjacent the screen, the filament generating heat whenenergized and heating the screen.

It is an object of this invention to provide a dark trace cathode-raytube of the foregoing character, including improved means for rapidlyheating the screen to obtain erasure.

Another object is to provide a tube of the foregoing character, whereinthe screen is rapidly and uniformly heated.

Still another object is to provide a tube of the foregoing character,wherein the heating means does not interfere either with the electronbeam which forms a trace on the screen or with a light source whichilluminates the trace on the screen.

Other objects and advantages of the invention will become apparent fromthe following description taken in conjunction with the accompanyinggures of the drawings, in which:

FIG. 1 is a sectional view of a tube embodying the invention;

FIG. 2 is an enlarged fragmentary view of a portion of the tube;

FIG. 3 is a still further enlarged fragmentary view of another portionof the tube;

l FIG. 4 is a sectional View taken on the line 4-4 of FIG. l;

FIG. 5 is an enlarged fragmentary view illustrating the operation of thetube;

FIG. 6 is a fragmentary View showing an alternative form of a portion ofthe tube; and

FIG. 7 is a view partially in section of still another alternative form.

In general, a dark trace cathode-ray tube in accordance with theinvention comprises a bulb which forms a sealed enclosure, the bulbincluding an elongated neck portion and a relatively large front faceportion. An electron gun is mounted within the neck of the bulb and ascotophor screen is mounted within the bulb adjacent the face, in thepath of electrons emitted by the e1ectron gun. A conventional electronbeam deflect-or is also provided for y3,413,505 Patented Nov. 26, 1968iceV deflecting the beam in order to form a trace or image on thescreen, the trace being visible from the face of the bulb when the4screen is illuminated by a light source.

To erase a trace from the screen, a toroidal heating coil or winding ispositioned within the enclosure, coaxially with the bulb and between theelectron gun and the screen. Behind the coil is provided an annularreector which refiects heat generated by the coil toward the screen. Therefiector and the coil are so constructed and are located relative toeach other and to the screen such that the screen is substantiallyuniformly heated at a rapid rate, and they are located radially outsideof the path of the electron beam.

The screen is preferably illuminated from its rearward side by anannular lamp positioned outside the bulb, and the heating coil and thereflector are also located out of the path of light rays leaving thelamp and striking the screen.

Since the heating coil and the refiector are out of the path of both theelectron beam and 'the light rays, they do not leave a shadow on thescreen either when a trace is being formed or when the trace is beingviewed.

In FIGS. l to 4, a dark trace tube embodying the invention comprises asealed envelope 10 which may be a glass bulb of the character commonlyused in cathoderay tubes. The envelope 10 comprises a relatively long,

cylindrical neck portion 11a, an enlarged, cylindrical front portion11C, and a funnel portion 11b connecting the portions 11a and 11c. Thefront of the envelope forms a face 18, through which an image may beviewed as will be described later.

For purposes well known in the art, a coating 21 of an electricallyconductive reflective material such as aluminurn is preferably formed onthe interior surface of the envelope, extending from the face 18 toapproximately the midway point of the funnel 11b, and another coating 22of an electrically conductive material such as colloidal graphite ispreferably formed on the interior surface of the envelope extending fromthe coating 21 into the neck 11a. The coatings 21 and 22 meet along aline indicated by the numeral 25 (FIG. 1). At approximately the junctureof the funnel 'and front portions of the envelope, an annular portion ofthe envelope is left uncoated, thus forming a window 25a, the purpose ofwhich will be explained hereinafter.

Mounted within the envelope are an electron gun 12 for developing afocused electron "beam 13, a scotophor screen assembly 19, and an eraseassembly 20 which includes a source of infra-red radiation. The gun 12is located in the neck Iportion 11a while the screen assembly 19 ispositioned in the front portion 11e such that it may be viewed throughthe face 18 of the envelope. Conventional beam deflecting apparatus isalso provided which may comprise either electrostatic means orelectromagnetic means. In the present instance, coils 14 of anelectromagnetic deflection system are shown. At the rear or base 16 ofthe envelope 10 a plurality of prongs 17 are provided for the purpose ofmaking electrical connections with some of the electrical componentswithin the envelope 10, such as the electron gun 12. To electricallyconnect the coatings 21 and 22 to an external circuit, a plurality ofconventional electrical connectors are provided which extend through andare hermetically sealed to the wall of the envelope. In the presentinstance, four connectors 23 are provided at the front portion 11c ofthe envelope and one connector 24 is provided in the funnel portion 11bof the envelope. The connector 24 is forward of the line 25 and iselectrically connected to the coating 22 by extending a portion of thecoating 22 up to the connector 24 and forming the coating over theportion of the connector 24 within the envelope. One of the fourconnectors 23 is similarly connected to the coating 21 and to the frameof the screen assembly 19 while the other connectors 23 are insulatedfrom the coating 21 and the screen assembly. In addition to makingelectrical connections between the external circuit and the screenassembly 19 and the erase assembly 20, the connectors 23 also serve tosupport the assemblies 19 and 20. The external circuit suitablyinterconnects the coatings 21 and 22 and the other components of thetube.

With reference to FIG. 2, the screen assembly 19 comprises a disk shapedscreen 31 which is illustrated in detail in FIG. 3. The screen 31comprises a transparent layer 32 of a material such as mica which servesas a support, a layer 33 of a scotophor material, and a layer 34 of aheat radiation absorption material such as carbon black. Since theabsorption layer 34 is at the front side of the screen and the screen isviewed from the front of the tube, the layer 34 must of course besufficiently thin that a trace on the layer 33 may be viewed through theface 18.

The screen 31 is positioned in a plane which is normal to the axis ofthe envelope 10, and is supported by an annular frame 36 which isL-shaped in cross section. One arm 36a (FIG. 2) of the L extendssubstantially in the plane f the screen 31 and over the front side ofthe screen 31 adjacent its edge, while the other arm 36b of the Lextends rearwardly across the edge of the screen 31. The screen 31 isheld in place against the arm 36a by an annular support member 37 whichextends into the opening of the L-shaped frame and over the back side ofthe screen 31 adjacent its edge. Between the members 36 and 37 ispositioned an annular cushion 38 of a compressible material such as acrumpled sheet of aluminum. Where the construction of the screen 31 issuch that it includes an electrically conductive material, the aluminumcushion 38 also serves to make an electrical connection between thescreen 31 and the frame member 36. The members 36 and 37, the screen 31,and the cushion 38 are held in assembled relation by, for example, aplurality of circumferentially spaced clips 39 which are secured to theouter surface of the arm 36b as by welding, and are bent inwardly overthe back side of the member 37.

The frame member 36 of the screen assembly 19 is supported within theenvelope by four contact fingers 41 and a plurality of spring fingers42, the fingers 41 and 42 being secured as by welding to the outersurface of the arm 36b and extending rearwardly therefrom. The contactfingers 41 include clips or clamps 43 at the rearward ends thereof,which are fastened to the connectors 23. At their inner ends, theconnectors 23 include inwardly extending prong-like members 44 which areengaged by the clips 43. The spring fingers 42 engage the inner surfaceof the envelope 10 and hold the screen assembly centered within theenvelope, and they also electrically connect the screen assembly withthe coating 21.

As is well known in the art, when the electron beam 13 strikes thescreen 31, opacity centers are formed in the scotophor layer 33. Toerase these centers, the erase assembly includes a heating coil 51 forgenerating heat and a reflector 52 for reflecting the heat to the screenassembly 31. The coil 51 is toroidal in shape and is located generallyat the focal line of the reflector 52, which is annular and generallyparabolic in cross section. The reflector 52 includes a radially innerwall 52a, the inner edge of which forms a circular opening 52b, and aradially outer wall 52C (FIGS. 4 and 5).

The reflector 52 and the coil 51 are supported by the screen assembly31. To this end, a plurality of circumferentially spaced metal rods 53are secured, as by welding, to the member 37 and extend rearwardlytherefrom. At their rearward ends, the rods 53 are secured together byan annular metal brace 54, and a plurality of circumferentially spacedrearwardly extending metal arms 56 secure the reflector 52 to the brace54. The foregoing connections may be made as by welding or rivets.

The coil 51 is fastened to the metal reflector S2 by a plurality ofcircumferentially spaced support wires 57. Each wire 57 is secured tothe bottom portion of the reflector 52 by forming a hole through thereflector 52, positioning one end of the wire 57 through the hole withthe wire out of contact with the reflector 52, and bonding the wire tothe reflector 52 with an electrical insulating material 58. The otherend of the wire 57 is twisted around a loop of the coil 51.

To pass current through the coil 51, one end 61 of the coil 51 isinsulated from the reflector 52 but is connected by a wire 62 to one ofthe clips 43, this particular clip being indicated by the numeral 43a.The clip 43a is fastened to its associated contact finger 41 by a strip63 of insulating material, whereby the clip 43a, the wire 62 and theassociated end of the coil 51 are electrically insulated from the framesof the screen and erase assemblies and from the coatings 21 and 22 butare electrically connected to the connector 23 associated with the clip43a. This particular connector is not the one which is connected to thecoating 21 and the frame of the screen assembly.

The other end of the coil 51 is electrically connected to the erase andscreen frames by an extension 64 of One of the arms 56 and through theframes to the connectors 43 other than the connector 43a. Thus, currentmay flow from the connector 43a, the wire 62, through the coil 51, theextension 64, the frame of the screen-assembly, and the connectors 43,in order to energize the coil and thereby heat the screen 31, theexternal power circuit preferably being such that the coil 51 may beenergized only when an image or trace is not being formed on the screen.

The reflector 52 is provided to increase the rate at which the screen isheated and to insure that the screen is uniformly heated. As shown inFIG. 5, each section of the-reflector directs heat to substantially allportions of the screen 31. While the coil 51 is at substantially thefocal line of the parabolic reflector 52, the coil 51 has considerablethickness and consequently it extends around the focal line. The heatrays emitted by the coil are therefore spread uniformly over the screen,resulting in rapid and uniform heating of the screen.

Since the screen 31 does not produce light, some lighting means must beprovided to light the screen for viewing purposes, and it is preferredthat a back lighting arrangement be used. In the present instance, acircular fluorescent lamp 66 (FIG. 1) is positioned coaxially with theenvelope around the funnel portion 11b. The lamp 66 is located adjacentthe window 25a, and consequently light shines through the window 25a,and illuminates the screen 31. If it is desired to decrease the amountof light entering the window 25a, a portion of the window 25a may-beblocked out by paint on the outer surface of the envelope. From FIG. 1it will be noted that the reflector 52 is located rearwardly to asuflicient extent relative to the window that the reflector does notcast a shadow on the screen. Further, from FIGS. 1 and 5 it will benoted that the electron beam 13 passes through the center of thereflector 52 and that the center opening 52b of the reflector issufficiently large that it does not interfere with the beam 13 evenunder maximum deflection conditions.

The radially outer side portion 52C of the reflector may be madeconsiderably longer than the radially inner side 52a and the outer sidehelps to spread the heat across the screen.

To increase the tendency of the reflector of the erase means to spreadthe heat rays uniformly over the screen, a tube may be providedincluding a reflector 67 (FIG. 6) having a plurality of dimples `68 onits inner surface, the dimples tending to scatter light.

The funnel portion of the glass envelope may be molded and coated toform a parabolic reflector. With reference to FIG. 7, a tube 70 is shownincluding an envelope 71 wherein a portion of the funnel of the envelopeis molded to form a parabola 72. A highly reflective coating 73 isformed on the inner surface of the parabola, and a heating coil 74 ismounted substantially at the center of the parabola. A screen assembly76 is provided which may be similar to the assembly 19, and the coil 74is supported by arms 77 which extend from the screen assembly 76 to thecoil 74. Again the coil 74 andthe reflector are designed to rapidly anduniformly heat the screen 76. In this form of the invention, the coating73 forms a reflector which is parabolic because of the shape of theenvelope.

It will be apparent from the foregoing that a novel and useful darktrace cathode-ray tube has been provided. The construction and locationof the erase assembly improve the performance and life of the screen ofthe tube. The reflector directs substantially all of the heat to thescreen, and is far more efficient than a tube wherein any reflection ofthe heat is from the tube walls and is therefore scattered throughoutthe tube. The erasure time of a tube embodying the invention is lessthan one-half that of a conventional tube.

The inner surface of the reflector should of course be bright and shinyto improve the reflection characteristics. While the reflector willordinarily be silver colored, it could be plated with gold, if thepreferred reflection is in the yellow region of the spectrum.

I'he construction of the scotophor screen is described in greater detailin the copending application of E. R.

Ewald et al., Ser. No. 554,024, filed May 31, 1966, now abandoned.

We claim:

1. A dark trace cathode-ray tube comprising an envelope, an electron gunin said envelope for developing a beam of electrons, a screen in saidenvelope extending substantially normal to and in the path of saidelectron beam, said screen including a layer of scotophor material whichdevelops opacity centers when said electron beam impinges thereon, andmeans for erasing said centers, said erasing means comprising agenerally annular source of infrared radiation positioned within saidenvelope rearwardly of said screen, and an annular generally parabolicreflector located adjacent to and generally rearwardly of said source ofradiation, said source of radiation and said parabolic reflector beinggenerally coaxial with said envelope and having a relatively largecenter opening through which said electron beam passes, said source ofradiation being located generally at the focal point of said reflector,whereby rearwardly emitted heat radiation from said source is reflectedtowards said screen and consequently substantially all of the heatradiation emitted by said source is spread uniformly over said screen.

2. Apparatus as in claim 1, and further including a light source forilluminating said screen, said light source being positioned adjacent tobut out of the path of light from said light source to said screen,whereby said erasing means does not cast a shadow on said screen.

3. Apparatus as in claim 1, wherein said source of radiation and saidreflector are positioned between said electron gun and said screen butout of the path of said electron beam, said reflector being separatefrom and mounted within the inner periphery of said envelope.

4. Apparatus as in claim 1, wherein said reflector is dimpled in orderto more uniformly spread the heat over said screen.

5. Apparatus as in claim 3, wherein said reflector and said source 'ofradiation are substantially annular, said Cil reflector comprising areflective coating on a parabolically shaped portion of said envelope.

6. A dark trace cathode-ray tube comprising an envelope having neck,funnel, and front portions, an electron gun mounted in said neck portionfor emitting a beam of electrons, a plurality of lingers secured to saidfront portion on the inside of said envelope, a frame fastened to saidlingers within said envelope, a screen `assembly mounted on said framein said front portion and extending substantially transversely of theaxis of said envelope, said screen assembly including a layer ofscotophor material which develops opacity centers when said electronbeam impinges thereon, and erase means mounted on said frame within saidenvelope adjacent said funnel portion, said erase means comprising anannular heater and an annular reflector located generally coaxiallywithin said envelope, said reflector being generally parabolic andlocated relative to said heater and to said screen assembly to reflectheat from said heater toward said screen assembly.

7. Apparatus as in claim 6, wherein said reflector comprises a metalmember, and said heater comprises a coil which is mounted generally atthe focal line of said reflector.

8. Apparatus as in claim 7, wherein the circuit path for electriccurrent flowing through said coil includes said reflector.

9. Apparatus as in claim 7, wherein the radially outer side of saidreflector is relatively long and extends forwardly adjacent the innersurface of said envelope.

10. A dark trace cathode-ray tube comprising an envelope having neck,funnel and front portions, an electron gun mounted in said neck portionfor emitting a beam of electrons, a screen assembly mounted in saidfront portion and extending substantially transversely of the axis ofsaid envelope, said screen assembly including a layer of scotophormaterial which develops opacity centers when said electron beam impingcsthereon, and erase means within said envelope adjacent said funnelportion, said erase means comprising an annular heater and an annularreflector located generally coaxially within said envelope, saidreflector ibeing generally parabolic and located relative to said heaterand to said screen assembly to reflect heat from said heater toward saidscreen assembly, said reflector comprising a metal member which issupported within said envelope, and said heater comprises a coil whichis mounted generally at the focal line of said reflector, the innerperiphery of said envelope adjacent said reflector being coated with areflective material, an annular portion of said envelope being uncoatedand thereby forming an annular window, and a circular lamp positionedoutside of said envelope adjacent said window, said reflector beinglocated sufficiently away from said window that light may shine throughsaid window and illuminate said screen assembly `and said reflector doesnot cast a shadow.

References Cited UNITED STATES PATENTS 1,676,300 7/1928 Sved 313--1132,387,038 10/1945 Owens 313-113 X 2,755,404 7/ 1956 Levy 313-912,775,407 7/ 1956 Fyler 313-2 JAMES W. LAWRENCE, Primary Examiner. V.LAFRANCHI, Assistant Examiner.

